1. Field of the Invention
This invention relates to a process for carbonylating olefinic compounds with the aid of cobalt carbonyl catalysts.
2. Description of the Prior Art
The reaction generally called "carbonylation," which includes the so-called hydroformylation for converting olefinic compounds into aldehydes and/or alcohols by reaction with a hydrogen/carbon monoxide gas mixture in the presence of a cobalt carbonyl catalyst and the so-called hydroesterification for converting olefinic compounds into esters by reaction with carbon monoxide and an alcohol, has so far been used widely in the industry. In this carbonylation, for example in the production of butyraldehydes from propylene, the reaction is carried out in the presence of a cobalt carbonyl catalyst under such reaction conditions as temperatures of 130.degree.-180.degree. C. and pressures of 100-300 atmospheres. Such high temperatures and high pressures, however, are undesirable from a commercial viewpoint because not only high cost of equipment and high cost of operation result but also formation of byproducts is significant.
In the conventional carbonylation processes using cobalt carbonyl catalysts, the products are separated from the reaction mixtures in a complicated manner; for example, prior to distillation, the cobalt carbonyl catalyst is separated from the reaction mixture in the form of metallic cobalt or a cobalt salt by steaming or by contacting with an aqueous mineral acid or an aqueous alkali solution and only thereafter the product is recovered by distillation. The reasons why which such a complicated procedure is required in separating the product from the reaction mixture are (1) that the cobalt carbonyl catalysts are unstable against heat under the conditions encountered in the distillation where the temperature is high and the carbon monoxide partial pressure is low, which makes it impossible to recycle the catalyst due to thermal decomposition of the cobalt carbonyl catalyst and deposition of metallic cobalt on the distillation vessel wall when the reaction mixture is subjected directly to distillation and (2) that, when the product aldehydes are distilled in the presence of the cobalt carbonyl catalyst, the aldehydes undergo undesirable side reactions which consequently decrease the yield of the aldehydes. Thus, the steps of separating and regenerating the catalyst are essential in the conventional processes, and so far the immediate distillation of the reaction mixture containing the catalyst has not been commercially employed. Moreover, conversion of the metallic cobalt or cobalt salt so recovered into an active cobalt carbonyl catalyst, requires severe conditions such as high temperature and high pressure. Thus, the difficulties of the principal reaction which requires high temperatures and high pressures in order to maintain the activity of the cobalt carbonyl catalyst under the conditions of carbonylation are further compounded by the catalyst regeneration process.
To reduce the instability of the cobalt carbonyl catalyst under low carbon monoxide partial pressure and also to make it possible to separate the product from the reaction mixture by distillation of the latter containing the catalyst, a method has been proposed which employs a cobalt cataylst modified with a trisubstituted phosphine, typically tributylophosphine or trioctylphosphine, as can be found in the production of butyl alchol and/or 2-ethylhexanol from propylene. One example of such a method may be found in J. Organometal. Chem., 13, 469 (1968). According to this method, the reaction can be carried out under lower pressures as compared with the case of the butyraldehydes synthesis from propylene with the aid of the above-mentioned cobalt carbonyl catalyst, and moreover the reaction mixture can, in principle, be subjected to distillation without any prior treatment. However, this method using such modified cobalt catalysts is not satisfactory in respect of catalyst separation and catalyst recycling; in fact, metallic cobalt is formed by decomposition of a part of the modified cobalt catalyst at the time of separation of the product from the reaction mixture and is deposited in the separation vessel or later, and part of the cobalt catalyst may accompany the distillate.
In view of the above, it is no exaggeration to say that the carbonylation of olefinic compounds with the aid of cobalt carbonyl catalysts, viewed from an industrial standpoint is brought with difficulty because of the problems of: (1) separation of cobalt catalysts from the reaction mixture, (2) reuse of the catalysts by recycling, and (3) the severity of the reaction conditions inevitably brought thereby.